Black polyester films are produced industrially, comprised of carbon-black-filled polyesters, and have been described in U.S. Pat. No. 4,865,898. These films are used inter alia as non-transparent layers in composites, or elsewhere for decorative purposes.
Carbon black can achieve low transparency and a high degree of blackness especially when nanoparticulate carbon blacks or graphites are used.
However, carbon blacks and graphites generally, and in particular in nanoparticle form, have two serious disadvantages in the production of stretched polyester films. Firstly, they are electrically conductive, a consequence of this being drastically increased risk of flashovers in the electrostatic lay-on systems generally used (short circuits or discharges between the electrostatic lay-on equipment and the chill roll, passing through the film), and secondly the lubricating properties of the carbon blacks and graphites give rise to very poor purging behavior. This means that the purging time on changeover from black film to other types of film in production operations is longer than usual. Both disadvantages have an adverse effect on economic factors in the productivity of black films.